What would be a use for such a crossmap? It would allow to quickly mashup data
from various projects in interesting ways. For example to show images of
species from EOL API on phylo-trees generated using Open Tree of Life API.

OpenTree Taxonomy has mapping of OTT IDs to NCBI IDs.
Encyclopdia of Life also has mapping of NCBI IDs to EOL IDs. So if
someone wants to map NCBI names to EOL using the same algorithm as EOL used –
they only need to query data about IDs. Even better – it would create a very
fast connection of one aggregator (Open Tree), to another aggregator (EOL)
through IDs of other sources without doing explicit name resolution.

Such queries would be much faster, as they would be just about comparing
indexed columns in a table. However the quality of the results from such
approach would depend on the quality of name resolution used by the
aggregators.

I am thinking about trying just that. As a pilot – we can generate Darwin Core
Archive files from OTT and EOL which would contain information about IDs from
other sources. Then we will need to add an API that would make it possible to
run queries from such information.

Another good suggestion from @jhpoelen – is to publish data about this kind of
crossmap as a csv file, that can be easily put in some kind of a database and
used separately on its own.